This invention relates to video data transmission and is especially useful in the transmission of video information employing conditional replenishment.
This application is related to commonly assigned co-pending U.S. application to Clapp Ser. No. 516,707 filed July 25, 1983.
A television frame typically contains about 200,000 picture elements, and if the data are to be sent digitally desirably 8 bits should be provided to convey the brightness information at each element. With a frame repetition frequency of 25 per second for a moving picture, digital transmission on the above basis would call for a transmission channel capacity of 40 M bit/s. This requirement can be reduced considerably by making use of the high correlation between one frame and the next which arises because usually only a small portion of the picture will be moving at any one time. Conditional replenishment involves the transmission of only the changes to an accuracy of only, say, 16 nonlinearly distributed quantising levels from one frame to the next and the resulting data reduction is normally greater than the additional data which must be sent to identify accurately the particular areas of the frame to which the changes relate. The areas are identified by line number and picture element address along the line; although 9 bits are necessary to identify the line number completely a saving is made by using only 3 bits giving the number modulo 8. A field synchronising code is sent to identify the first line of a field and so are all line numbers whether there is a change in the line or not. The rate of data transmission needed using conditional replenishment varies considerably because it depends on the amount of the frame which is moving at the particular time, whereas it is much more convenient to have a constant data transmission rate. In order to overcome this difficulty buffer stores are provided at both ends of the transmission channel, and it is important that these buffer stores neither become empty nor overflow. Using this technique it has proved possible to transmit a moving picture satisfactorily over a 2 M bit/s channel.
If B.sub.E (t) is the number of bits stored in the encoder buffer (termed the encoder state) at the transmission end of the channel at a time t and B.sub.D (t) is the number of bits stored in the decoder buffer at the recpetion end of the channel at time t, it can be shown that EQU B.sub.E (t-.DELTA.t)+B.sub.D (t)=V.sub.R .multidot..DELTA.t
where V.sub.R is the transmission capacity (in bit/s) of the channel and is assumed to be constant and .DELTA.t is the time delay between data entering the encoder buffer store and them leaving the decoder buffer store. Normally an optimum value for .DELTA.t is chosen which makes the total of the data stored in the buffers equal to half of the total available buffer memory, and the control of the decoder buffer is based on the state of the encoder buffer at a time .DELTA.t earlier. This means that the emptying and overflowing of the decoder buffer can be anticipated and appropriate action taken, either temporarily suspending decoding or discarding data, so that the corruption of the reproduced picture resulting from data suddenly being not available or being lost can be avoided.
Since the replenishment data is derived from an image scanned by a conventional television raster and the reproduced image is updated from the replenishment data line by line of a similar raster, it follows that the reading of data from the decoder buffer must remain in track with the writing of data into the encoder buffer if changes in the first image are to appear accurately in the reproduced image. This forms an additional constraint on the reading from the decoder buffer and is handled by transmitting the encoder buffer state B.sub.E (t) to the decoder so that the decoder buffer state B.sub.D (t) can be predicted on the basis of the equation given above.
In addition, four techniques are used to reduce the extremely high rate of video data which might otherwise occur when there is a lot of picture movement. These four techniques are:
(1) The sensitivity of the movement detector at the encoder is reduced as the encoder buffer fill increases so that the number of areas of detected picture change is reduced. PA1 (2) Field sub-sampling on a time basis is introduced. In this technique alternate fields of information are discarded and the decoder is operated so as to interplate the missing information from the two adjacent transmitted fields. PA1 (3) Sub-sampling on an element basis is introduced so that certain moving area elements are discarded by the encoder and interpolated by the decoder. PA1 (4) If the above techniques fail to prevent imminent overfill of the encoder buffer, then all movement of the transmitter picture is ignored until the encoder buffer state has returned to a safe value.
The reduction of the sensitivity of the movement detector at the encoder has the effect of freezing low contrast change information which results in an effect known as "dirty window". This can be tolerated for short periods. Field sub-sampling produces a movement jerkiness over the whole field and the provision of an interpolator at the decoder can be used to reduce the jerkiness by assuming that all movement is taking place at a uniform speed. Element sub-sampling which involves the transmission of the changes relating to alternate elements, for example, enables the amount of data transmitted when there is a lot of picture movement to be reduced to a minimum without introducing the stop movement technique referred to in (4) above. A disadvantage of element sub-sampling is that in areas of picture detail interpolation can be objectionable because the interpolation assumes that the brightness of an interpolated point is the average of the two adjacent points in the same line. This quite clearly is not true when the interpolated point is on a vertical edge between light and dark areas.